A Switchable Ferrocene-Based [1]Rotaxane with an Electrochemical Signal Output

An Interlocked Figure-of-Eight Molecular Shuttle

Here is reported the synthesis and characterization of an interlocked figure-of-eight rotaxane molecular shuttle from a dibenzo-24-crown-8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self-entanglement through the complexation of the DB24C8 moiety. In the targeted figure-of-eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation-then-carbamoylation of the ammonium. This way, two discrete co-conformational states were obtained, in which the folding and size of the two loops could be changed.

Dual and Sequential Locked/Unlocked Photochromic Effects on FRET Controlled Singlet Oxygen Processes by Contracted/Extended Forms of Diarylethene-Based [1]Rotaxane Nanoparticles

Manipulations of singlet oxygen (¹ O₂ ) generations by the integration of both aggregation-induced emission luminogen (AIEgen) photosensitizer and photochromic moieties have diversified features in photodynamic therapy applications. Through Förster resonance energy transfer (FRET) pathway to induce red PL emissions (at 595 nm) for ¹ O₂ productions, [1]rotaxane containing photosensitive tetraphenylethylene (TPE) donor and photochromic diarylethene (DAE) acceptor is introduced to achieve dual and sequential locked/unlocked photoswitching effects by pH-controlled shuttling of its contracted/extended forms. Interestingly, the UV-enabled DAE ring closure speeds follow the reversed trend of DAE self-constraint degree as: contracted < extended < noninterlocked forms in [1]rotaxane analogues, thus FRET processes can be adjusted in contracted/extended forms of [1]rotaxane upon UV irradiations. Accordingly, the contracted form of [1]rotaxane is FRET-OFF locked and inert to UV exposure due to the larger bending conformation of DAE parallel (p-)conformer, compared with its extended and noninterlocked analogues possessing switchable FRET-OFF/ON behaviors activated by dual and sequential pH- and photoswitching. Owing to the advantages of ¹ O₂ productions tuned by multistimuli inputs (pH, UV, and blue light), an useful logic circuit for toxicity outputs of the surface modified [1]rotaxane nanoparticles (NPs) has been demonstrated to offer promising ¹ O₂ productions and managements based on mechanically interlocked molecules for future bioapplications.

Dynamic [1]rotaxanes via a reversible covalent bond and host-guest anion recognition

Bambus[6]uril-based [1]rotaxanes were formed quantitatively, utilizing a bis(acyloxy)iodate(I) reversible covalent bond and host-guest anion recognition. These novel [1]rotaxanes exhibited a dynamic nature facilitating carboxylate component exchange reactions in acetonitrile.

Helic[6]arene-Based Chiral Pseudo[1]rotaxanes and [1]Rotaxanes

Chiral pseudo[1]rotaxanes and [1]rotaxanes constructed from macrocyclic arenes still remain a big challenge mainly owing to the lack of such chiral macrocycles. In this work, a new system of chiral pseudo[1]rotaxanes formed by self-inclusion of helic[6]arene containing amide linked with the terminal tertiary amines was first discovered. Based on an atom-economic stopping strategy, a pair of chiral [1]rotaxanes were conveniently obtained in almost quantitative yields by blocking the pseudo[1]rotaxanes with monobenzyl bromide of tetraphenylethene. The structures of pseudo[1]rotaxanes and [1]rotaxanes were characterized by 2D NMR spectra in solution, combined with DFT calculations. The photophysical properties further revealed the efficient chirality transfer of helic[6]arene to the tetraphenylethene moiety, compared to their unthreaded chiral isomers. The discovery of the chiral pseudo[1]rotaxanes allows for a wide and available synthesis of chiral [1]rotaxanes, and also opening a new avenue to the design of chiral supramolecular materials.

Rapid and High-Yield Synthesis of [23]Crown Ether: Applied as a Wheel Component in the Formation of Pseudo[2]rotaxane and Synthesis of [2]Catenane with a Dibenzylammonium Dumbbell

A facile, rapid, and high yield synthesis of [23]crown ether (X23C7) has been developed from commercially available starting materials, in one step with good to excellent yield. The reaction is completed in 6 h under room temperature conditions, with the highest yield being 81%. The X23C7 macrocycle formed pseudo[2]rotaxane with a dibenzylammonium ion (DBA⁺) dumbbell, exhibiting strong association (K_a = 2.61 × 10³ M^-1). Consequently, a [2]catenane was synthesized from a DBA⁺-based diolefin terminated salt and X23C7 in 81% yield, using a threading-followed-ring-closing-metathesis approach.

Switching the solubility of electroactive ionic liquids for designing high energy supercapacitor and low potential biosensor

Ionic liquids are regarded as one of the most prodigious materials for sustainable technological developments with superior performance and versatility. Hence, in this study, we have reported the design and synthesis of electroactive disubstituted ferrocenyl ionic liquids (Fc-ILs) with two different counter anions and demonstrated the significance of their anion tuneable physicochemical characteristics towards multifunctional electrochemical applications. The Fc-IL synthesized with chloride counter anion (Fc-Cl-IL) displays water-solubility and can be used as a redox additive in the fabrication of supercapacitor. Supercapacitor device with Fc-Cl-IL based redox electrolyte exhibits outstanding energy and power densities of 91 Wh kg^-1 and 20.3 kW kg^-1, respectively. Meanwhile, ferrocenyl IL synthesized with perchlorate anion (Fc-ClO₄-IL) exhibits water-insolubility and can serve as a redox mediator towards construction of a glucose biosensor. The biosensor comprising Fc-ClO₄-IL is able to detect glucose at an exceptionally lower potential of 0.2 V, with remarkable sensitivity and selectivity. This study implies that the introduction of electroactive ILs could afford supercapacitor devices with high energy and power densities and biosensors with less detection potential.

An amide hydrogen bond templated [1]rotaxane displaying a peptide motif - demonstrating an expedient route to synthetic mimics of lasso peptides

The rapid synthesis of an amide hydrogen bond templated [1]rotaxane is reported - demonstrating a potential pathway to synthetic analogues of lasso peptides. The structures of the [1]rotaxane and its unthreaded isomer have been characterized by NMR spectroscopy and modelled using DFT calculations.

Pillar[5]arene Based [1]rotaxane Systems With Redox-Responsive Host-Guest Property: Design, Synthesis and the Key Role of Chain Length

Pillar[n]arenes are a new type of macrocyclic compounds, which were first reported in 2008 by Ogoshi. They not only have cylindrical, symmetrical, and rigid structures, but also have many advantages, including easy functionalization and rich host-guest properties. On the other hand, mechanically interlocked molecules (MIMs) exist extensively in nature which have been artificially synthesized and widely applied in the fields of nanotechnology and biology. Although pillar[5]arene-based MIMs have been investigated much over recent years, pillar[5]arene-based [1]rotaxanes are very limited. In this report, we synthesized a series of amide-linked pillar[5]arene-based [1]rotaxanes with ferrocene unit as the stopper. Under the catalysis of HOBT/EDCL, the mono-amido-functionalized pillar[5]arenes were amidated with ferrocene carboxylic acid to constructed ferrocene-based [1]rotaxanes, respectively. The structure of the [1]rotaxanes were characterized by ¹H NMR, ¹³C NMR, 2D NMR, mass spectroscopy, and single-crystal X-ray structural determination. In the experiment, the monofunctionalized pillar[5]arene was synthesized with a self-inclusion property, which allows for forming a pseudo-rotaxane. The key role is the length of the imine chain in this process. The formation of a rotaxane was realized through amidation of ferrocene dicarboxylic acid, which acted as a plug. In addition, due to the ferrocene units, the pillar[5]arene-based [1]rotaxanes perform electrochemically reversible property. Based on this nature, we hope these pillar[5]arene-based [1]rotaxanes can be applied in battery devices in the future.

Synthesis of diamido-bridged bis-pillar[5]arenes and tris-pillar[5]arenes for construction of unique [1]rotaxanes and bis-[1]rotaxanes

The pillar[5]arene mono- and di(oxyalkoxy)benzoic acids were successfully prepared in high yields by sequential alkylation of ω-bromoalkoxy-substituted pillar[5]arenes with methyl or ethyl p-hydroxybenzoate followed by a hydrolytic reaction under basic conditions. Under catalysis of HOBt/EDCl, the amidation reaction of pillar[5]arene mono(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes afforded diamido-bridged bis-pillar[5]arenes. 1H NMR and 2D NOESY spectra clearly indicated that [1]rotaxanes were formed by insertion of longer diaminoalkylene unit into the cavity of one pillar[5]arene with another pillar[5]arene acting as a stopper. The similar catalysed amidation reaction of pillar[5]arene di(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes resulted in the diamido-bridged tris-pillar[5]arenes, which successfully form the unique bis-[1]rotaxanes bearing longer than diaminopropylene diamido bridges.

Ferrocene and Transition Metal Bis(Dicarbollides) as Platform for Design of Rotatory Molecular Switches

Design of rotatory molecular switches based on extremely stable sandwich organometallic complexes ferrocene and bis(dicarbollide) complexes of transition metals is reviewed. The "on"-"off" switching in these systems can be controlled by various external stimuli such as change of the solution pH, interactions with coordinating species or redox reactions involving the central atom or substituents in the ligands.

Pillar[5]arene-based [1]rotaxane: high-yield synthesis, characterization and application in Knoevenagel reaction

Quantitative synthesis of a pillararene-based [1]rotaxane has been achieved via a “self-threading-stoppering” approach, followed by its first application in organic catalysis.

Formation of a series of stable pillar[5]arene-based pseudo[1]-rotaxanes and their [1]rotaxanes in the crystal state

A series of mono-amide-functionalized pillar[5]arenes with different lengths of N-ω-aminoalkyl groups as the side chain on the rim were designed and synthesized, which all formed pseudo[1]rotaxanes in the crystal state. And these pseudo[1]rotaxanes could be transformed into [1]rotaxanes or open forms in the crystal state. In addition, they were also studied in solution by (1)H NMR spectroscopy.

Recognition of steric factor in external association of xanthenocrown-5 and bis-napthalenocrown-6 with bis(benzimidazolium)propane borontetrafluoride

This is the first report of charge transfer band for crown-axel H-bonding interaction in acetonitrile medium. Monte Carlo simulation established external association of small cavity crown with BBIM-propane dication axels. Resonance energy, binding constant and ratiometric detection of association of xanthenocrown-5 (1) and bis-napthalenocrown-6 (2) with bis-(benzimidazolium)propane borontetrafluoride (3a-3d) in acetonitrile determined the effect of steric factor towards association.

Ferrocene-containing [1]-, [2]-, [3]- and [4]rotaxanes synthesized from a common precursor

[1]-, [2]-, [3]- and [4]rotaxanes were synthesized by reaction of azide compounds with dialkylammonium with ferrocenyl and alkynyl terminal groups.

Synthesis of triazolium-based mono- and tris-branched [1]rotaxanes using a molecular transporter of dibenzo-24-crown-8

We report a diverted route to [1]rotaxane and tris-branched [1]rotaxane that are devoid of any efficient template and which could not be obtained by classical straightforward strategies. The described chemical route relies on the utilization of a "macrocycle transporter", which is able first to bind a macrocycle, second to link temporarily a triazolium-containing molecular axle, and third to deliver the macrocycle around the new docked axle through molecular machinery in a [1]rotaxane structure. The extended encircled thread is eventually cleaved by an amine or a triamine to afford the triazolium-containing [1]rotaxanes, releasing at the same time, the macrocycle transporter as a recyclable species.

A fluorescent bistable [2]rotaxane molecular switch on SiO2 nanoparticles

A fluorescent bistable [2]rotaxane was immobilized onto the surface of SiO₂ nanoparticles through click reaction. The shuttling motion of the macrocycle component was driven by external acid–base stimuli both in solution and on SiO₂ nanoparticles, accompanied by visual fluorescence changes.

Linear–cyclic polymer structural transformation and its reversible control using a rational rotaxane strategy

A new strategy for “polymer structural transformation” was developed. One [1]rotaxane unit was introduced at the chain end of a linear polymer and the wheel component position was defined by controlling the attractive interaction between the polymer ends. Thus, the reversible linear–cyclic structural transformation was demonstrated.

A novel dynamic pseudo[1]rotaxane based on a mono-biotin-functionalized pillar[5]arene

A stable pillar[5]arene-based pseudo[1]rotaxane P1′ was synthesized by the click reaction, which exhibited a dynamic slow disassembly process upon adding a strong-polar solvent or competitive guest. Moreover, this dynamic behavior might be used as a switch to turn on or off the bioactivity of the biotin moiety in aqueous solution.

A switchable bis-branched [1]rotaxane featuring dual-mode molecular motions and tunable molecular aggregation

A multifunctional bis-branched [1]rotaxane containing a perylene bisimide (PBI) core and two identical bistable[1]rotaxane arms terminated with ferrocene units was prepared and characterized by (1)H NMR, (13)C NMR, and 2D ROESY NMR spectroscopies and by HR-ESI spectrometry. The system is shown to possess several key features: (1) In acetone solution, external acid-base stimuli can result in relative mechanical movements of its ring and thread, which can induce extension and contraction movements of the whole system accompanied by a rotational movement of the ferrocene units, thus realizing dual-mode molecular motions, and the optimized conformations at different states are obtained through molecular dynamics simulations employing the general Amber force field. (2) The introduction of PBI enables the system fluorescence encoding through distance-dependent photoinduced electron transfer process from the ferrocene units to the PBI fluorophore. (3) The addition of Zn(2+) can increase the degree of aggregation of the system, while adding base hinders aggregation because of the movement of the macrocycle. The tunable aggregated nanostructural morphologies of [1]rotaxane were examined by scanning electron microscopy. These results can pave the way to achieve precise control of integrated and coupling nanomechanical motions at a single-molecule level and provide more insight into controlling the aggregate behavior of switchable mechanically interlocked molecules.

Two switchable star-shaped [1](n)rotaxanes with different multibranched cores

Two novel star-shaped [1](n)rotaxanes with three and four identical [1]rotaxane arms but different multibranched cores were designed, synthesized, and well-characterized. In the two systems, external base-acid stimuli result in the uniform relative mechanical movement of the macrocyclic rings and threads of their [1]rotaxane arms. The energy-minimized structures of the two rotaxanes in different states were obtained using molecular dynamics simulations in acetone solution, suggesting the construction of more sophisticated molecular machines mimicking the extension and contraction motions.

A musclelike [2](2)rotaxane: synthesis, performance, and molecular dynamics simulations

A novel bistable symmetric [2](2)rotaxane was prepared by a threading-followed-by-stoppering strategy and characterized with (1)H, (13)C, and 2D ROESY NMR spectroscopy and HR-ESI spectrometry. The symmetric [2](2)rotaxane system consists of an anthracene-based bis(crown ether) as macrocycles, and each of the two dibenzo[24]crown-8 (DB24C8) rings is threaded by the pendant substituents of a symmetrically substituted central rotatable ferrocene subunit that possesses two distinguishable recognition sites for the DB24C8 ring: namely, a dibenzylammonium site and an N-methyltriazolium site. The uniform shuttling motion of the thread relative to the two DB24C8 rings in [2](2)rotaxane can be driven by external acid-base stimuli, which was evidenced by (1)H and 2D ROESY NMR spectroscopy. Furthermore, molecular dynamics simulations of the [2](2)rotaxane were carried out both in protonated (stretched) and in neutral (contracted) forms. The calculated percentage change in molecular length of the [2](2)rotaxane between the two end-capping bis(methoxyl)phenyl groups is about 48% in the two different states (in acetone), which is much larger than the percentage change (∼27%) in human muscle. Moreover, in the two states, the C*-Cp-Cp-C* dihedral angles are computed as -177° in the stretched state and -112° in the contracted state, indicating a correlation between the translational and rotational motions of the [2](2)rotaxane.

Fluorescence modulation in tribranched switchable [4]rotaxanes

Two novel tribranched [4]rotaxanes with a 1,3,5-triphenylene core and three rotaxane arms have been designed, synthesized, and characterized by (1)H and (13)C NMR spectroscopies and HR-ESI mass spectrometry. [4]Rotaxanes 1 and 2 each possess the same three-armed skeleton. Each arm incorporates two distinguishable binding sites for a dibenzo[24]crown-8 ring, namely a dibenzylammonium site and an N-methyltriazolium site, and is terminated by a 4-morpholino-naphthalimide fluorophore as a stopper. [4]Rotaxane 1 has three di-ferrocene-functionalized dibenzo[24]crown-8 rings whereas 2 has three simple dibenzo[24]crown-8 rings interlocked with the thread component. Uniform shuttling motions of the three macrocycles in both 1 and 2 can be driven by external acid-base stimuli, which were confirmed by (1)H NMR spectroscopy. However, [4]rotaxanes 1 and 2 show distinct modes of fluorescence modulation in response to external acid-base stimuli. [4]Rotaxane 1 exhibits a remarkable fluorescence decrease in response to the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base, which can displace the ferrocene-functionalized macrocycle from the dibenzylammonium station to the N-methyltriazolium station. In contrast, the fluorescence intensity of [4]rotaxane 2 showed an enhancement with the addition of DBU. Time-resolved fluorescence measurements have been performed. The different photoinduced electron-transfer processes responsible for the fluorescence changes in the two molecular systems are discussed. Topological structures of this kind have significant potential for the design and construction of large and complex assemblies with controllable functions.